My truck died, Im stuck on what to look for next as the problem !?

OK, so my turck died on me while I was going down the road, figure I would ask on here to see if anybody has any ideas of what it could be wrong. i was driving about 45mph and I notice that my truck all of a sudden dies on me and everything but the radio cuts off, so I get it off the road and raise the hood on it, I just put an alternator on it about a week and a half ago, it was from a junkyard but it was charging fine, had it tested, so I checked all of my battery terminal and they were tight and check the wires going to the alternator, checked fuses in the engine bay and the inside of the truck, everything checked out fine, no service engine lights or anything, truck also has a brand new battery as well, I even tapped on the started to see if that was gone bad although I know it would cause the truck to cut off going down the road while running, I can go to put the key in it and it will turn over like a half a turn then stopkind of like its not got enough juice to start even the though battery is charged and everything, so Im kind of stuck on what to try next any ideas?? I would really appreciate it thanks